Compressed air is supplied from the main to port 1 and, in the position of spool 7 shown, both ports, 2 and 3, connected to the ends of the operating cylinder or device, are closed off. When pilot air under pressure is admitted through port 5, piston 6, spool 7 and piston 8 begin to move to the left and piston 8 shifts sleeve 9, overcoming the resistance of spring 10. This motion continues until sleeve 9 runs up against cover 11. After spool 7 has been thus shuttled, air from port 1 is admitted into port 2, and port 3 is connected to port 4, which leads to the atmosphere. If port 5 is connected to the atmosphere, sleeve 9, actuated by spring 10, shifts piston 8 returning spool 7 to its central position. Accurate positioning of the spool is accomplished because the motion of sleeve 9 to the right is limited by bushing 12, and piston 6 is located by sleeve 14. If pilot air is admitted through port 13, spool 7 is shuttled so that air from port 1 is admitted into port 3, and port 2 is connected to the atmosphere through port 4.
$3679$SHP,FC$